ACTIVE ANTENNA ARRAY LINEARIZATION

LINEARISATION DE RESEAU D'ANTENNES ACTIVES

English Abstract

Systems and methods for linearizing a radio system are disclosed. In some embodiments, a radio system comprises an antenna array, transmit branches comprising respective power amplifiers, a predistortion subsystem comprising predistorters for the transmit branches respectively, a receive antenna element, a transmit observation receiver having an input coupled to the receive antenna element, and an adaptor. The predistorters predistort respective transmit signals to provide predistorted transmit signals to the respective transmit branches for transmission via respective active antenna elements in the antenna array. The transmit observation receiver is operable to receive, via the receive antenna element, a combined receive signal due to coupling between the receive antenna element and the active antenna elements. The adaptor is operable to generate a combined reference signal based on the transmit signals and configure predistortion parameters input to the predistorters based on the combined reference signal and the combined receive signal.

French Abstract

La présente invention concerne des systèmes et des procédés pour linéariser un système radio. Dans certains modes de réalisation, un système radio comprend un réseau d'antennes, des branches de transmission comprenant des amplificateurs de puissance respectifs, un sous-système de prédistorsion comprenant des dispositifs de prédistorsion respectivement pour les branches d'émission, un élément d'antenne de réception, un récepteur d'observation d'émission dont une entrée est couplée à l'élément d'antenne de réception, et un adaptateur. Les dispositifs de prédistorsion prédéforment les signaux d'émission respectifs pour fournir des signaux d'émission prédistordus aux branches d'émission respectives pour une transmission par l'intermédiaire d'éléments d'antenne active respectifs dans le réseau d'antennes. Le récepteur d'observation d'émission est utilisable pour recevoir, par l'intermédiaire de l'élément d'antenne de réception, un signal de réception combiné dû au couplage entre l'élément d'antenne de réception et les éléments d'antenne active. L'adaptateur est apte à générer un signal de référence combiné sur la base des signaux de transmission et pour configurer des paramètres de prédistorsion entrés dans les dispositifs de prédistorsion sur la base du signal de référence combiné et du signal de réception combiné.

Claims

23
Claims
What is claimed is:
1. A radio system (100), comprising:
an antenna array (106) comprising a plurality of active antenna elements
(108);
a plurality of transmit branches (102) comprising a respective plurality of
power amplifiers (104), the plurality of transmit branches (102) operable to
transmit a plurality of predistorted transmit signals via the plurality of
active
antenna elements (108), respectively;
a predistortion subsystem comprising a plurality of predistorters (110) for
the plurality of transmit branches (102) respectively, the plurality of
predistorters
(110) operable to predistort a respective plurality of transmit signals to
provide
the plurality of predistorted transmit signals and provide the plurality of
predistorted transmit signals to the plurality of transmit branches (102),
respectively;
a receive antenna element (114);
a transmit observation receiver (116) having an input coupled to the
receive antenna element (114), the transmit observation receiver (116)
operable
to receive, via the receive antenna element (114), a combined receive signal
that
corresponds to a combination of the plurality of transmit signals received at
the
receive antenna element (114) due to coupling between the receive antenna
element (114) and the plurality of active antenna elements (108); and
an adaptor (112) operable to:
generate a combined reference signal based on the plurality of
transmit signals such that the combined reference signal models the
combined receive signal; and
configure predistortion parameters input to the plurality of
predistorters (110) that define predistortion provided by the plurality of
predistorters (110) based on the combined reference signal and the
combined receive signal.

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2. The radio system (100) of claim 1 wherein the adaptor (112) is operable
to
configure the predistortion parameters input to the plurality of predistorters
(110)
that define predistortion provided by the plurality of predistorters (110)
based on
the combined reference signal, the combined receive signal, and known complex
valued attenuation factors that define the coupling from the plurality of
active
antenna elements (108) to the receive antenna element (114).
3. The radio system (100) of claim 2 wherein the predistortion parameters
are a common set of predistortion coefficients for the plurality of
predistorters
(110).
4. The radio system (100) of claim 2 wherein:
the predistortion parameters are an estimated predistortion coefficient
vector, ak+1, that defines a common set of predistortion coefficients for the
plurality of predistorters (110); and
in order to configure the predistortion parameters, the adaptor (112) is
further configured to compute the estimated predistortion coefficient vector,
ak+l,
in accordance with:
<IMG>
where
= ak is a prior set of predistortion parameters for the plurality of
predistorters (110) used to generate the plurality of predistorted
transmit signals;
= n is a scaling convergence factor;
= 13, is a coupling factor between the n-th active antenna element (108)
and the receive antenna element (114);
= .x., is the transmit signal that is predistorted by the respective
predistorter (110) to provide the predistorted transmit signal for the
transmit branch (102) for the n-th active antenna element (108);

25
= yk is the combined receive signal; and
<IMG>
where
<IMG>
5. The radio system (100) of claim 2 wherein the predistortion parameters
comprise a separate set of predistortion coefficients for each of the
plurality of
predistorters (110).
6. The radio system (100) of claim 1 wherein the adaptor (112) is operable
to
configure the predistortion parameters input to the plurality of predistorters
(110)
that define predistortion provided by the plurality of predistorters (110)
based on
the combined reference signal, the combined receive signal, and one or more
estimated parameters that take into consideration unknown complex valued
attenuation factors that define a coupling from the plurality of active
antenna
elements (108) to the receive antenna element (114).
7. The radio system (100) of claim 6 wherein the predistortion parameters
are a common set of predistortion coefficients for the plurality of
predistorters
(110).
8. The radio system (100) of claim 6 wherein the predistortion parameters
comprise a separate set of predistortion coefficients for each of the
plurality of
predistorters (110).
9. The radio system (100) of any one of claims 1 to 8 wherein the receive
antenna element (114) is a dedicated antenna element for the transmit
observation receiver (116).

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10. A method of operation of a radio system (100) to linearize the radio
system (100), comprising:
predistorting (500) a plurality of transmit signals via a respective plurality
of predistorters (110) of the radio system (100) to thereby provide a
plurality of
predistorted transmit signals;
transmitting (502) the plurality of predistorted transmit signals via a
respective plurality of active antenna elements (108) in an antenna array
(106) of
the radio system (100);
receiving (504) a combined receive signal via a receive antenna element
(114) and a transmit observation receiver (116) of the radio system (100),
wherein the combined receive signal corresponds to a combination of the
plurality of transmit signals received at the dedicated antenna (108) due to
coupling between the receive antenna element (114) and the plurality of active
antenna elements (108);
generating (506) a combined reference signal based on the plurality of
transmit signals such that the combined reference signal models the combined
receive signal; and
configuring (508), based on the combined reference signal and the
combined receive signal, predistortion parameters input to the plurality of
predistorters (110) that define predistortion provided by the plurality of
predistorters (110).
11. The method of claim 10 wherein configuring (508) the predistortion
parameters comprises configuring (508) the predistortion parameters based on
the combined reference signal, the combined receive signal, and known complex
valued attenuation factors that define a coupling from the plurality of active
antenna elements (108) to the receive antenna element (114).
12. The method of claim 11 wherein the predistortion parameters are a
common set of predistortion coefficients for the plurality of predistorters
(110).

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13. The method of claim 11 wherein:
the predistortion parameters are an estimated predistortion coefficient
vector, ak+1, that defines a common set of predistortion coefficents for the
plurality of predistorters (110); and
configuring (508) the predistortion parameters comprises computing the
estimated predistortion coefficient vector, ak+1, in accordance with:
<IMG>
where
= ak is a prior set of predistortion parameters for the plurality of
predistorters (110) used to generate the plurality of predistorted
transmit signals;
= n is a scaling convergence factor;
= 13, is a coupling factor between the n-th active antenna element (108)
and the receive antenna element (114);
= .x., is the transmit signal that is predistorted by the respective
predistorter (110) to provide the predistorted transmit signal for a
transmit branch for the n-th active antenna element (108);
= yk is the combined receive signal; and
<IMG>
where
14. The method of claim 11 wherein the predistortion parameters comprise a
separate set of predistortion coefficients for each of the plurality of
predistorters
(110).

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15. The method of claim 10 wherein configuring (508) the predistortion
parameters comprises configuring (508) the predistortion parameters based on
the combined reference signal, the combined receive signal, and one or more
estimated parameters that take into consideration unknown complex valued
attenuation factors that define a coupling from the plurality of active
antenna
elements (108) to the receive antenna element (114).
16. The method of claim 15 wherein the predistortion parameters are a
common set of predistortion coefficients for the plurality of predistorters
(110).
17. The method of claim 15 wherein the predistortion parameters comprise a
separate set of predistortion coefficients for each of the plurality of
predistorters
(110).
18. The method of any one of claims 10 to 17 wherein the receive antenna
element (114) is a dedicated antenna element for the transmit observation
receiver (116).

Description

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ACTIVE ANTENNA ARRAY LINEARIZATION
Technical Field
The present disclosure relates to linearization of a power amplifier in a
radio system and more specifically relates to linearization of multiple power
amplifiers coupled to multiple active antenna elements in a radio system.
Background
Any form of Radio Frequency (RF) transmit unit will have restrictions on
how much extra spectrum emission is allowed outside its own transmit
bandwidth. Different requirements may be placed on the RF transmit unit
depending on where in the spectrum domain that the transmit bandwidth of the
RF transmit unit appears. As an example, in-band emissions and out-of-band
emissions are two of the many possible requirements. Mostly, out-of-band
emissions can be effectively controlled by applying RF filters. Also,
filtering of
the in-band spectrum might actually be possible from a purely theoretical
point of
view. However, the filtering of the in-band spectrum is usually never
considered
because such filtering would severely limit the use of the hardware equipment
to
a specific carrier frequency and could not be retuned even within the RF
transmit
unit's operating frequency band. As such, since the power amplifier of the RF
transmit unit is usually responsible for the emissions, designers typically
try to
linearize the amplifier instead.
Linearization can be performed in several ways, both in analog form and
in digital form. In analog form, linearization is usually performed at RF, but
could
possibly also be made at low frequency or Intermediate Frequency (IF). Digital
linearization is usually performed at digital baseband. In its simplest form,
digital
linearization predistorts the signal before it enters the power amplifier in
such a
way that the predistortion more or less cancels out the distortion produced by
the
power amplifier. This technique is called Digital Predistortion (DPD).
However,
.. feed forward approaches also exist where the distortion caused by the power
amplifier is cancelled directly at the output of the power amplifier. Feed
forward

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approaches have drawbacks both in terms of design complexity and inherently
weak power efficiency.
It is clear that in most cases some kind of linearization is often needed to
be implemented. In general, there is a one-to-one mapping between
predistorters and amplifiers. That is, hardware for a specific predistorter is
implemented and designed for each amplifier. In conjunction to the activator
functionality of the predistorter itself, a feedback system is usually
designed and
implemented to keep track of the actual output signal and to be able to react
to
changes in the system that may need changes in the activator in turn.
Issues arise when extending DPD to so-called Active Antenna System
(AAS) and Multiple Input Multiple Output (MIMO) (e.g., massive MIMO) systems
where there are many active antenna radio branches. Today, the number of
antenna branches, and thus the number of power amplifiers, is on the order of
hundreds. Using existing DPD technology, a separate predistorter and feedback
loop is needed for each separate power amplifier. For each power amplifier,
the
feedback loop uses a linearizer algorithm that tries to minimize the extra
intermodulation spectrum that is produced by the power amplifier by in turn
configuring the respective predistorter to add an appropriate amount of signal
distortion to the input.
A typical implementation includes a predistorter, an amplifier, a coupler, a
receiver that is dedicated to the distortion detection, and an adaptor that
adaptively configures the predistorter to minimize the distortion in the
feedback
signal. The coupler takes a small amount of the RF signal, which is fed back
to
the adaptor via the receiver where it is compared to the original signal. One
example would be to feed the feedback signal through an Analog to Digital
Converter (ADC) and then transfer it back to digital complex baseband by means
of an In-phase and a Quadrature phase component (I and Q) where it is
compared to the original signal. The adaptor applies an algorithm that
typically
tries to minimize the difference between the original signal and the output
signal
from the amplifier except for possibly a gain factor and an optional phase +
time
lag difference. This kind of loop is then repeated for every branch in the
active

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antenna radio system. This results in a massive amount of hardware and
processing for linearization.
Particularly in radio systems such as AAS and MIMO systems having a
large number of active antenna radio branches, there is a need to drastically
reduce the amount of hardware needed to simultaneously linearize many
amplifiers while at the same time reduce signal processing needs.
One way of reducing the hardware needed is to use one of the receive
branches as a feedback system for the linearization, as described in [1]. That
is,
no special receptor for the feedback signal has to be implemented. This will
work
if the type of communication system is a Time Domain Division (TDD), but not
for
a Frequency Domain Division (FDD) system. Moreover, special control signaling
would be needed to switch some of the branches in transmit mode, while one or
possibly more branches would be set into receive mode.
Linearization of a full active array antenna requires careful design and
calls for any means by which complexity and hardware need can be reduced.
Summary
Systems and methods for linearizing a radio system comprising an
antenna array are disclosed. In some embodiments, a radio system comprises
an antenna array comprising a plurality of active antenna elements, a
plurality of
transmit branches comprising a respective plurality of power amplifiers, a
predistortion subsystem comprising a plurality of predistorters for the
plurality of
transmit branches respectively, a receive antenna element, a transmit
observation receiver having an input coupled to the receive antenna element,
and an adaptor. The predistorters are operable to predistort respective
transmit
signals to provide predistorted transmit signals and provide the predistorted
transmit signals to the respective transmit branches. The transmit branches
are
operable to transmit the predistorted transmit signals via the respective
active
antenna elements. The transmit observation receiver is operable to receive,
via
the receive antenna element, a combined receive signal that corresponds to a
combination of the transmit signals received at the receive antenna element
due

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to coupling between the receive antenna element and the active antenna
elements. The adaptor is operable to generate a combined reference signal
based on the transmit signals such that the combined reference signal models
the combined receive signal and configure predistortion parameters input to
the
predistorters that define predistortion provided by the predistorters based on
the
combined reference signal and the combined receive signal. In this manner,
linearization for each of the branches of the radio system can be performed
without the need for couplers for each antenna element and with low processing
complexity.
In some embodiments, the adaptor is operable to configure the
predistortion parameters input to the predistorters that define predistortion
provided by the predistorters based on the combined reference signal, the
combined receive signal, and known complex valued attenuation factors that
define the coupling from the active antenna elements to the receive antenna
element. Further, in some embodiments, the predistortion parameters are a
common set of predistortion coefficients for the predistorters. In some
embodiments, the predistortion parameters are an estimated predistortion
coefficient vector (ak+i) that defines a common set of predistortion
coefficients
for the predistorters and, in order to configure the predistortion parameters,
the
adaptor is further configured to compute the estimated predistortion
coefficient
vector, ak+i, in accordance with:
N
ak+i = ak + n = All',- = [(I A, .x) - ykl
n =1
where
= ak is a prior set of predistortion parameters at iteration k for the
plurality of predistorters used to generate the plurality of predistorted
transmit signals;
= n is a scaling convergence factor;
= ign is a coupling factor between the n-th active antenna element and
the receive antenna element;

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= .x., is the transmit signal that is predistorted by the respective
predistorter to provide the predistorted transmit signal for the transmit
branch for the n-th active antenna element;
= Yk is the combined receive signal; and
5
M: = 11- ci = it 4 k) ¨ 1 = M lici
where
Mk = [Yk Yk = lYkl2 Yk = lYk14... ' ' ']=
In some other embodiments, the predistortion parameters comprise a separate
set of predistortion coefficients for each of the predistorters.
In some embodiments, the adaptor is operable to configure the
predistortion parameters input to the predistorters that define predistortion
provided by the predistorters based on the combined reference signal, the
combined receive signal, and one or more estimated parameters that take into
consideration unknown complex valued attenuation factors that define a
coupling
from the active antenna elements to the receive antenna element. In some
embodiments, the predistortion parameters are a common set of predistortion
coefficients for the predistorters. In some other embodiments, the
predistortion
parameters comprise a separate set of predistortion coefficients for each of
the
predistorters.
In some embodiments, the receive antenna element is a dedicated
antenna element for the transmit observation receiver.
Embodiments of a method of operation of a radio system are also
disclosed. In some embodiments, a method of operation of a radio system to
linearize the radio system comprises predistorting a plurality of transmit
signals
via a respective plurality of predistorters of the radio system to thereby
provide a
plurality of predistorted transmit signals and transmitting the predistorted
transmit
signals via respective active antenna elements in an antenna array of the
radio
system. The method further comprises receiving a combined receive signal via a
receive antenna element and a transmit observation receiver of the radio
system,

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wherein the combined receive signal corresponds to a combination of the
transmit signals received at the dedicated antenna due to coupling between the
receive antenna element and the active antenna elements. The method further
comprises generating a combined reference signal based on the transmit signals
such that the combined reference signal models the combined receive signal and
configuring, based on the combined reference signal and the combined receive
signal, predistortion parameters input to the predistorters that define
predistortion
provided by the predistorters.
In some embodiments, configuring the predistortion parameters comprises
configuring the predistortion parameters based on the combined reference
signal, the combined receive signal, and known complex valued attenuation
factors that define a coupling from the active antenna elements to the receive
antenna element. In some embodiments, the predistortion parameters are a
common set of predistortion coefficients for the predistorters. In some
embodiments, the predistortion parameters are an estimated predistortion
coefficient vector (ak+1) that defines a common set of predistortion
coefficents for
the plurality of predistorters and configuring the predistortion parameters
comprises computing the estimated predistortion coefficient vector (ak+1) in
accordance with:
[ N
ak+i = ak + n = mi',- = (1 ign = xn) ¨ Ykl
[ n =1
where
= ak is a prior set of predistortion parameters at iteration k for the
plurality of predistorters used to generate the plurality of predistorted
transmit signals;
= n is a scaling convergence factor;
= ign is a coupling factor between the n-th active antenna element and
the receive antenna element;
= xn is the transmit signal that is predistorted by the respective
predistorter to provide the predistorted transmit signal for a transmit
branch for the n-th active antenna element;

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= yk is the combined receive signal; and
M: = (0 = mk)-1- = Mr
where
Mk = [Yk Yk. lYkl2 Yk. lYk14... ''']=
In some other embodiments, the predistortion parameters comprise a separate
set of predistortion coefficients for each of the predistorters.
In some embodiments, configuring the predistortion parameters comprises
configuring the predistortion parameters based on the combined reference
signal, the combined receive signal, and one or more estimated parameters that
take into consideration unknown complex valued attenuation factors that define
a
coupling from the plurality of active antenna elements to the receive antenna
element. In some embodiments, the predistortion parameters are a common set
of predistortion coefficients for the predistorters. In some other
embodiments,
the predistortion parameters comprise a separate set of predistortion
coefficients
for each of the predistorters.
In some embodiments, the receive antenna element is a dedicated
antenna element for the transmit observation receiver.
Brief Description of the Drawings
The accompanying drawing figures incorporated in and forming a part of
this specification illustrate several aspects of the disclosure, and together
with
the description serve to explain the principles of the disclosure.
Figure 1 illustrates a radio system that provides linearization in
accordance with some embodiments of the present disclosure;
Figure 2 illustrates an adaptation scheme utilized by the adaptor in the
radio system of Figure 1 in accordance with some embodiments of the present
disclosure;

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Figure 3 illustrates a radio system that provides linearization in
accordance with some other embodiments of the present disclosure;
Figure 4 illustrates a radio system that provides linearization in
accordance with some other embodiments of the present disclosure; and
Figure 5 illustrates a flow chart that illustrates a linearization process in
accordance with some embodiments of the present disclosure.
Detailed Description
The embodiments set forth below represent information to enable those
skilled in the art to practice the embodiments and illustrate the best mode of
practicing the embodiments. Upon reading the following description in light of
the
accompanying drawing figures, those skilled in the art will understand the
concepts of the disclosure and will recognize applications of these concepts
not
particularly addressed herein. It should be understood that these concepts and
applications fall within the scope of the disclosure.
Linearization of a full active array antenna requires careful design and
calls for any means by which complexity and hardware need can be reduced. In
this regard, there are some existing solutions that try to minimize the
necessary
hardware by re-using the same linearizer feedback receiver by switching it
around all of the amplifier branches which have their own coupler interface
[2].
This would usually be called Transmit Observation Receiver (TOR) sharing.
TOR sharing requires some clever scheduler, the couplers, and a switch
network. A method [3] is described that utilizes one common linearizer for all
antenna branches in an active array antenna. It takes the summed-up power in a
"power sensor" and uses that information to control the biasing of the
different
amplifiers, the gain of the amplifiers, and also the shaping of the common
linearizer as to give linearization.
The problem with the existing solution described in [2] is that it requires a
significant amount of hardware in terms of Radio Frequency (RF) couplers and
possibly transmission line routing on the antenna board itself, and it
requires full
signal processing capability to cope with a full set of linearizers, one for
each

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branch of the antenna array. If it is not the case of TOR sharing, a separate
TOR
has to be implemented for each antenna branch. In addition, there would be a
signal processing loop for each antenna branch with its separate algorithm
controlling an individual linearizer.
The solution in [3] uses a power detector and is able to control power
settings of the different amplifiers, together with the individual branch gain
and
also the shape of the common linearizer. This solution lacks the ability to
linearize a multitude of beams in the array antenna. It has only one
linearizer for
all of the active antenna branches at the same time. If digital beamforming is
to
be used, then individual linearizers have to be used for each amplifier in the
active array antenna. An active array antenna supporting several beams at the
same time (digital beamforming) needs to have a linearizer dedicated to each
amplifier branch.
These linearizers could be all the same, e.g., if all amplifiers are the same,
or may be differently shaped if each amplifier is to be linearized separately.
The
algorithm for the whole multitude of linearizers may be given the task of
picking
some average parameter set for the linearizer, or it may be given the task of
individually linearizing each amplifier.
Systems and methods are disclosed herein that drastically reduce the
amount of hardware needed to simultaneously linearize multiple amplifiers and
at
the same time save signal processing needs in the same order of pace. The
systems and methods disclosed herein have a clear application towards
implementing linearization of an active array antenna where all of the antenna
elements are separately driven by individual power amplifiers.
More specifically, in some embodiments a radio system is provided that
includes an antenna array (e.g., an Active Antenna System (AAS)) including
multiple antenna elements, transmit branches coupled to the antenna elements
respectively, a TOR (also referred to herein as a linearizer receiver), and a
receive antenna element (e.g., an additional antenna element that is dedicated
for the TOR). The antenna elements are referred to herein as "active antenna
elements" because the respective transmit branches include Power Amplifiers

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(PAs). The receive antenna element may be positioned arbitrarily somewhere
nearby the antenna array. In addition, the radio system includes a
predistortion
subsystem that includes a separate digital predistorter or linearizer for each
transmit branch and an adaptor that configures predistortion parameters input
to
5 the digital predistorters to control the predistortion applied by the
digital
predistorters.
In operation, transmit signals for the active antenna elements are
predistorted by the respective digital predistorters to provide predistorted
transmit
signals, which are provided to the respective transmit branches for
transmission
10 via the respective active antenna elements. Due to coupling between the
active
antenna elements and the receive antenna element, the TOR receives a
combined receive signal via the receive antenna element. This combined signal
is a combination of the transmit signals transmitted via the active antenna
elements. The adaptor generates a combined reference signal based on the
transmit signals such that the combined reference signal models (i.e.,
emulates)
the combined receive signal received via the TOR. The adaptor generates (e.g.,
updates) the predistortion parameters provided to the digital predistorters
based
on the combined receive signal and the combined reference signal. For
example, the combined receive signal and the combined reference signal may be
compared after one or both has been adjusted such that the two signals are
time
and phase aligned, e.g., to determine an error. An error minimization
technique
(e.g., Least Mean Squares (LMS)) can then be used to update the predistortion
parameters such that the error between the combined receive signal and the
combined reference signal is minimized. The difference between the emulated
signal combination and the actually measured signal would be a figure of merit
of
the algorithm convergence. Because the radio system uses a separate linearizer
for each transmit branch, rather than a common linearizer, the radio system is
able to perform simultaneous linearization for multiple beams at the same
time,
while also reducing processing complexity.
While not being limited to or by any particular advantages, embodiments
of the present disclosure provide a number of advantages. For example, the

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embodiments disclosed herein avoid the need for RF couplers on each antenna
(amplifier) branch and an RF routing network from the couplers down to the TOR
receiver(s). As such, the amount of hardware needed is significantly reduced.
Further, the signal processing needed is reduced to the equivalent of what is
needed for a single linearizer loop. Embodiments disclosed herein also provide
an efficient way to linearize simultaneous beams by so-called digital
beamforming by the same hardware.
The embodiments disclosed herein provide a solution for applying
linearization to a large active antenna array that promises a large reduction
in
implementation cost, while also reducing the complexity of the linearization
system to a large extent.
Another advantage of some embodiments of the present disclosure is that
an existing linearizer solution in terms of the algorithm by which an optimum
solution may be found for single antenna linearization may be re-used. So,
there
.. is no need for designing any new linearizer algorithm. Rather, new inputs
to an
existing linearization algorithm are used to thereby obtain an output having a
new
combined meaning.
Figure 1 illustrates one example of a radio system 100 in accordance with
some embodiments of the present disclosure. The radio system 100 may be, for
example, implemented in a base station of a cellular radio access network
(e.g.,
a Fifth Generation (5G) New Radio (NR) radio access network). As illustrated,
the radio system 100 includes multiple transmit branches 102-1 through 102-6
(also referred to herein as antenna branches) including respective PAs 104-1
through 104-6 coupled to an antenna array 106, which in this example is an
AAS.
The PAs 104-1 through 104-6 of the transmitter branches 102-1 through 102-6
are coupled to respective antenna elements 108-1 through 108-6. The antenna
elements 108-1 through 108-6 are also referred to herein as active antenna
elements 108-1 through 108-6. While not illustrated, the transmit branches 102-
1
through 102-6 may include additional components such as, e.g., Digital to
Analog
Converters (DACs), upconverters, filters, and/or the like. Note that the
transmit
branches 102-1 through 102-6 are generally referred to herein collectively as

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transmit branches 102 and individually as transmit branch 102. Likewise, the
PAs 104-1 through 104-6 are generally referred to herein collectively as PAs
104
and individually as PA 104, and the antenna elements 108-1 though 108-6 are
generally referred to herein collectively as antenna elements 108 and
individually
as antenna element 108. Also note that while there are six transmit branches
102 and six antenna elements 108 in this example, the number of transmit
branches 102 and antenna elements 108 can be any integer number greater
than or equal to 2.
The radio system 100 also includes a linearization subsystem, which in
this example is a Digital Predistortion (DPD) subsystem. Note that while DPD
is
used to provide linearization in many of the example embodiments described
herein, it should be noted that other types of linearization may be used
(e.g.,
analog predistortion). The DPD subsystem includes separate DPD predistorters
110-1 through 110-6 (also referred to herein as DPD actuators) for the
respective
transmit branches 102-1 though 102-6 and an adaptor 112, which may be
implemented in hardware or a combination of hardware and software. A
feedback loop provides a feedback signal (yFB) that is used by the adaptor 112
to
generate (e.g., update) predistortion parameters input into the DPD
predistorters
110 based on the feedback signal.
In this example, it is assumed that all of the PAs 104 have exactly the
same non-linear behavior. In this case, all of the DPD predistorters 110 are
configured with the same set of predistortion parameters (e.g., the same set
of
complex valued predistortion coefficients). Having a separate DPD predistorter
110 for each transmit branch 102 enables each PA 104 to be provided with a
different signal that is to be amplified. Importantly, it should be understood
that
the transmit signals may be all different, and it is the individual PAs 104
that are
to be linearized and not the transmit signals themselves. So, regardless of
the
transmit signals, the predistortion is only changed if the non-linear behavior
of the
PAs 104 for some reason changes.
The feedback loop includes a receive antenna element 114 (also referred
to herein as a feedback antenna element) and a TOR 116. The TOR 116

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includes typical receiver components such as, e.g., a Low Noise Amplifier
(LNA),
filter(s), downconversion circuitry, and in some cases Analog to Digital
Converter
(ADC) circuitry. Preferably, the receive antenna element 114 is a dedicated
antenna element, i.e., an antenna element that is dedicated for the TOR 116.
.. The receive antenna element 114 is positioned near the antenna elements
108.
In operation, transmit signals x1 through x6 for the transmit branches 102-1
through 102-6 for the respective active antenna elements 108-1 through 108-6
in
the antenna array 106 are provided to the respective DPD predistorters 110-1
through 110-6. The DPD predistorters 110-1 through 110-6 predistort the
transmit signals x1 through x6 to thereby provide respective predistorted
transmit
signals x1' through x6', which are provided to the respective transmit
branches
102-1 through 102-6 for transmission via the active antenna elements 108-1
through 108-6, respectively.
During transmission of the predistorted transmit signals x1' through x6', the
.. TOR 116 outputs a feedback signal (yFB) that is received via the receive
antenna
element 114 due to a coupling (i.e., an antenna-to-antenna coupling) from each
of the active antenna elements 108-1 through 108-6 to the receive antenna
element 114. The feedback signal (yFB) is referred to herein as a combined
receive signal or a combined feedback signal because it is the combination of
the
.. predistorted transmit signals x1' through x6' transmitted via the active
antenna
elements 108-1 through 108-6 received at the receive antenna element 114 due
to the coupling from each of the active antenna elements 108-1 through 108-6
to
the receive antenna element 114. These couplings are denoted by respective
coupling parameters 131 through 136. Importantly, these couplings do not
require
hardware couplers and, as such, hardware is reduced as compared to existing
solutions that require hardware couplers for each antenna element. In this
example, the coupling parameters 131 through 136 are known. For example, the
coupling parameters 131 through 136 may have been previously measured during
calibration or estimated based on a physical distance between the antenna
elements 108 and the receive antenna element 114 (e.g., a Look Up Table (LUT)

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may be populated with estimated values for the different antenna elements 108
such that the needed values can be obtained from the LUT as needed).
The adaptor 112 generates a combined reference signal from the transmit
signals x1 through x6 such that the combined reference signal models (i.e.,
emulates) the combined receive signal (i.e., the feedback signal yFB). The
adaptor 112 utilizes an adaption scheme, or algorithm, to generate (e.g.,
update)
the predistortion parameters input to the DPD predistorters 110 based on the
combined receive signal and the combined reference signal. More specifically,
in
some embodiments, after time and phase aligning the combined refence signal
and the combined receive signal, an error between the combined receive signal
and the combined reference signal is determined. A minimization scheme (e.g.,
LMS) is utilized to generate the predistortion parameters such that this error
is
minimized.
In this manner, a single receive antenna 114 and a single TOR 116 are
used to obtain the combined receive signal that is representative of all of
the
transmit signals that are output by the PAs 104 in the active transmit
branches
(i.e., the transmit branches 102 that are coupled to the active antenna
elements
108). Further, a single adaptor 112 (i.e., a single adaptation algorithm) can
be
used to generate the predistortion parameters input into all of the DPD
predistorters 110. In other words, a single adaptor 112 is used for all PAs
104 for
all of the antenna elements 108 in the antenna array 110.
Figure 2 illustrates the adaptation scheme implemented by the adaptor
112 in accordance with some embodiments of the present disclosure. More
specifically, the adaptation scheme illustrated in Figure 2 is an LMS
algorithm to
minimize the error between the combined reference signal and the combined
receive signal, where 'y' is the combined receive signal (denoted as yFB in
Figure
1), 'r' is the combined reference signal, 'a' is the sought predistortion
parameters,
'MAT' is the derivative matrix, and 'q' is the scaling convergence factor. The
adaptation scheme is an iterative scheme where the error between the combined
receive signal and the combined reference signal is minimized in each step of
the
iteration. The speed of convergence is controlled by essentially evaluating
the

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multi-dimensional derivative (MAT) and in addition to the scaling factor q. By
starting with a guess of the solution (e.g., some default initial set of
predistortion
parameters), the solution may be iteratively improved by this search algorithm
which is actually an implementation of Newton's search method. The algorithm
5 gives an
LMS solution to the problem. The predistortion parameters, ak+i, for
the (k+1)-th iteration can also be expressed as:
n=i ak+i = ak +11 = MI lc- ' [(EN ign = xn) ¨ Yki (1)
where Yk is the k-th measurement of the combined receive signal, Mk is the
measurement matrix for the non-linear model of the PAs 104,
10 Wc = (Mr = MO-1 ' Mr, (2)
and
Mk = [Yk Yk . lYkl2 Yk . lYk14... ' ' ] (3)
The predistortion parameters ak (e.g., complex valued predistortion
coefficients)
are updated according to the iteration formula (Equation (1)) above.
15 Note that the above adaptation scheme is only an example. Variations to
the adaption scheme described above with respect to Figure 2 and Equations (1)
to (3) may also be used. Further, any other algorithm for minimizing the error
(e.g., in a LMS sense) between the combined receive signal and the combined
reference signal may be used.
It should also be noted that, in Figure 2, the solution is written in Matlab
notation where the T-character means LMS-solution. More specifically, the '\'-
character means Pseudoinverse in Matlab language, which is almost the same
thing as doing the LMS-procedure. Matlab has provided a convenient way of fast
calculation of the LMS, and that is essentially what the A' character means.
Equations (1) ¨ (3) also provide an LMS solution. In this regard, the
following
notes can be made with respect to Figure 2 and Equations (1) ¨ (3). The term
"YFB" is used in Figure 1 to denote the combination of the individual signals
coming from all of the active antenna elements 108 at the receive antenna 114.
In Figure 2, it can be said that yFB = y. Further, Y = 13 means the summation
over
all signals yn coming from the different antenna ports. 'Y' is the column
matrix of
all these signals. Also, one could say that Mit = MATk, but Mit = (MrMk)-1MH

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as being the Pseudoinverse to Mk. That is, it should be compliant with M = M+
=
/ (identity matrix), in a Least Means Square sense. Further, X =,3 in Figure 2
is
equivalent to the term Eirv,=1 13, = xn in Equation (1). It should also be
noted that
the exemplary adaptation scheme described above with respect to Figure 2 and
Equations (1) to (3) is outlined in general terms because the unknown
parameter
a may take on various forms. For example, in some embodiments, the
parameter a may express a single scalar, a vector of parameters, or even
entries
in a LUT. This may be freely set so as to reflect the behavior of the PAs 104
for
the desired implementation. In an even more general description of the LMS
.. solution, a single matrix inversion scheme may also be performed as to
avoid the
iterative implementation.
Thus, it should be understood that the example embodiment of the
adaptation scheme of Figure 2 and Equations (1) to (3) should merely be viewed
as an example and that the present disclosure is not limited thereto. The
present
.. disclosure focuses on the overall implementation of having just one single
receive antenna element 114 and TOR 116 without having the need to
implement RF couplers and/or a network routing tree for each of the different
feedback signals, in combination with a multitude of linearizers.
Now a number of additional embodiments are described that give
.. additional freedom to the linearization process described above. In this
regard,
Figure 3 illustrates the radio system 100 in accordance with another
embodiment
in which the assumption of all of the PAs 104 having equal non-linear behavior
is
lifted. Otherwise, the radio system 100 and the operation thereof is the same
as
that described above. However, since the PAs 104 may have different non-linear
characteristics but the same set of predistortion parameters is utilized by
all of
the DPD predistorters 110, linearization of each of the branches is no longer
provided. However, the combined receive signal will still get linearized. That
is,
although some of the PAs 104 still transmit a relatively high distortion
level, the
sum of the signal still gets linearized.
Figure 4 illustrates the radio system 100 in accordance with another
embodiment in which the DPD predistorters 110 are configured with different

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predistortion parameters. In other words, the adaptor 112 generates separate
predistortion parameters for the separate DPD predistorters 110. This
multiplies
the number of unknown parameters to solve for in the adaptor 112 and the
convergence ratio of the overall algorithm may be affected. However, it is
still
.. possible from an LMS view point to have all the linearizers dedicated to
each
own non-linear amplifier. In order to extend the algorithm of Figure 2 and
Equations (1) to (3) above to solve for separate coefficients for the DPD
predistorters 110, the coefficient vector 'alpha' describing the non-linearity
is N
times stacked to accommodate for extra coefficients for the additional
independent DPD predistorters 110. The gradient matrix M is also extended with
additional columns as to take care for the extra unknowns in the LMS
optimization procedure.
In yet another embodiment, the embodiment of any one of Figures 1 to 4
can be further extended to accommodate unknown coupling factors between the
antenna elements 108 and the receive antenna element 114. More specifically,
in one example embodiment, an uncertainty factor is introduced. For example,
the coupling parameters may be estimated based on, e.g., physical distance
between the antenna elements 108 and the receive antenna element 114 with
the addition of an uncertainty factor. This will disturb the convergence of
the
algorithm, but nevertheless it will arrive at a fairly stable solution. In
another
example embodiment, the coupling parameters are included into the LMS
solution as unknowns. Notably, in order to also give provisions for allowing
the
'betas' (i.e., the coupling parameters) to be unknown or at least uncertain,
one
can, in accordance with what is described above about extra DPD coefficients,
stack extra beta-unknowns below/above the original alpha vector. That is the
unknown iteration vector would be [alpha; beta]. The gradient matrix M is also
extended with extra columns corresponding to the derivative with respect to
each
beta. Then, the approach would be to iterate not only over the alphas, but
also
over the betas at the same time.
Lastly, it should be noted that while the DPD predistorters 110 and the
adaptor 112 are illustrated in Figures 1, 3, and 4 as being part of the radio

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system 100, in some other embodiments, the DPD predistorters 110 and/or the
adaptor 112 are implemented external to the radio system 100 such as, e.g., at
another device or system or virtualized "in the cloud." In this case, the
radio
system 100 would, e.g., receive the predistorted transmit signals from the
external device/system (e.g., the other device or "cloud") and provide the
combined receive signal to the external device/system where it is used to
generate (e.g., update) the predistortion parameters of the DPD predistorters
110. As another example, the adaptor 112 is implemented at an external
device/system but the DPD predistorters 110 are implemented at the radio
system 100, in which case the predistortion parameters input to the DPD
predistorters 110 would be received from the externa device/system.
Figure 5 is a flow chart that illustrates a linearization process in
accordance with at least some aspects of the embodiments described above.
This process is performed by a radio system such as, e.g., the radio system of
Figure 1, 3, or 4. As such, references to the radio system 100 are included in
the
description below. As illustrated, the process includes predistorting transmit
signals via respective predistorters 110 of the radio system 100 to thereby
provide predistorted transmit signals (step 500). The process further includes
transmitting the predistorted transmit signals via respective active antenna
elements 108 in the antenna array 106 of the radio system 100 (step 502). The
process further includes receiving a combined receive signal via the receive
antenna element 114 and the TOR 116 of the radio system 100, wherein the
combined received signal corresponds to a combination of the transmit signals
received at the dedicated antenna element 114 due to coupling between the
receive antenna element 114 and the active antenna elements 108 (step 504).
The process further comprises generating a combined reference signal based on
the transmit signals such that the combined reference signal models the
combined receive signal (step 506) and configuring, based on the combined
reference signal and the combined receive signal, predistortion parameters
input
to the predistorters that define predistortion provided by the predistorters
(step
508).

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In some embodiments, configuring the predistortion parameters comprises
configuring the predistortion parameters based on the combined reference
signal, the combined receive signal, and known complex valued attenuation
factors that define the coupling from the active antenna elements 108 to the
receive antenna element 114, as described above. In some embodiments, the
predistortion parameters are a common set of predistortion coefficients for
the
DPD predistorters 110.
As further described above, in some embodiments, the predistortion
parameters take the form of an estimated predistortion coefficient vector,
ak+i,
that defines a common set of predistortion coefficients for the DPD
predistorters
110, and configuring the predistortion parameters comprises computing the
estimated predistortion coefficient vector, ak+i, in accordance with:
[ N
ak+i = ak + n = mi',- = (I A, = xn) ¨ ykl
[ n =1
where
= ak is a prior set of predistortion parameters at iteration k for the DPD
predistorters 110 used to generate the predistorted transmit signals;
= n is a scaling convergence factor;
= ign is the coupling factor between the n-th active antenna element 108
and the receive antenna element 114;
= xn is the transmit signal that is predistorted by the respective DPD
predistorter 110 to provide the predistorted transmit signal for the
transmit branch 102 for the n-th active antenna element 108;
= Yk is the combined receive signal; and
M: = (Mr = Mk) -1 ' Mr
where
Mk = [Yk Yk . [Ykl2 Yk ' lYkl4"' "'1=

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As further described above, in some embodiments, the predistortion
parameters comprise a separate set of predistortion coefficients for each of
the
DPD predistorters 110.
As also described above, in some other embodiments, configuring the
5 predistortion parameters comprises configuring the predistortion
parameters
based on the combined reference signal, the combined receive signal, and
unknown complex valued attenuation factors that define the coupling from the
active antenna elements 108 to the receive antenna element 114. Further, in
some embodiments, the predistortion parameters take the form of a common set
10 of predistortion coefficients for all of the DPD predistorters 110. In
some other
embodiments, the predistortion parameters comprise a separate set of
predistortion coefficients for each of the DPD predistorters 110.
As further described above, in some embodiments, the receive antenna
element 114 is a dedicated antenna element for the TOR 116.
15 At least some of the following abbreviations may be used in this
disclosure. If there is an inconsistency between abbreviations, preference
should
be given to how it is used above. If listed multiple times below, the first
listing
should be preferred over any subsequent listing(s).
= 5G Fifth Generation
20 = AAS Active Antenna System
= ADC Analog to Digital Converter
= DAC Digital to Analog Converter
= DPD Digital Predistortion
= FDD Frequency Domain Division
= I and Q In-phase and Quadrature phase component
= IF Intermediate Frequency
= LMS Least Mean Squares
= LNA Low Noise Amplifier
= LUT Look Up Table
= MIMO Multiple Input Multiple Output
= NR New Radio

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= PA Power Amplifier
= RF Radio Frequency
= TDD Time Domain Division
= TOR Transmit Observation Receiver
Those skilled in the art will recognize improvements and modifications to
the embodiments of the present disclosure. All such improvements and
modifications are considered within the scope of the concepts disclosed
herein.

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References
[1] CA 2804444 Al, "Method and apparatus to use auxiliary receiver to
compensate multiple transmitters based upon one of the transmitters"
[2] EP 3 255 799 Al, "Reducing distortions in amplified signals radiated by a
multiple antenna system"
[3] US 20170163217 Al, "Simultaneous Linearization Of Multiple Power
Amplifiers With Independent Power," Publication date June 8, 2017

Representative Drawing